BIOTITE TINGUAITE DYKE ROCK. 61 



small crystals, in sufficient amount to give the rock its 

 greenish cast. Its crystallization preceded that of the 

 feldspars and the crystals are lounded or l)roken, irregu- 

 lar fragments. The sections occur from deep grass green 

 to almost colorless, and the deeper colored show a marked 

 pleochroism r^ =: bluish green, ?>=: grass green, c = green- 

 ish-yellow. The axis of greatest elasticit}^ lies nearest 

 to c and the extinction in most of the sections is practi- 

 cally parallel. 



Magnetite is prominent and marks the remains of rather 

 large plates of a former dark silicate. Most of the orig- 

 inal silicate has completely disappeared, leaving only the 

 patches of black oxide of iron, but in an occasional sec- 

 tion, a greenish-brown silicate still remains between the 

 black borders of magnetite, which from its absorption, 

 parallel extinction and characteristic shimmer, is evidently 

 biotite. From the similarity of the sections, it is reason- 

 able to assume that they were all originally this biotite, 

 and if so it must have been a biotite very poor in mag- 

 nesia, since so little of this oxide occurs in the rock. 



Sodalite is seen in small purplish to colorless isotropic 

 sections of low refraction, some showing dodecahedral 

 cleavage lines. A few small crystals of apatite and zir- 

 con occur as inclusions in the feldsi)ars. 



The tinguaite dike at Pickard's Point, Manchester, 

 originally described by Sears, ^ has been shown by Wash - 

 ingtcm- to contain much analcite and he classifies the tin- 

 giiaites of this locality as analcite tinguaites. Very little 

 isotropic mineral occurs in the dike described here and 

 from its appearance and the presence of chlorine what is 

 present is judged to be sodalite, so the dike can hardly 

 be classed with the one he describes. 



' J. H. Sears, Bull. Essex Inst. XXV, 4, 1893. 



' H. 8. Washington, Am. Jour. Sci. vl, 1898, p. 176. 



